This invention relates to a method for controlling ignition timing in an internal combustion engine. More particularly, the invention relates to a method for controlling ignition timing in an internal combustion engine provided with an exhaust gas recirculation device.
In order to reduce the contents of harmful components in the exhaust gas of an internal combustion engine and to obtain lower fuel consumption, it is necessary to effectively control the spark-advance of the engine in response to the engine's operating conditions. Since this optimum spark-advance angle changes depending mainly on the rotational speed of the engine and on the value of the absolute pressure in the intake manifold of the engine, namely, on the load of the engine, according to the conventional technique, a governor advance mechanism is used for setting the engine speed advance angle and a vacuum advance mechanism is used for setting the engine load advance angle. An apparatus is also used together with a digital computer for electronically controlling the engine speed advance angle and the engine load advance angle.
In an internal combustion engine provided with an exhaust gas recirculation device (hereinafter referred to as an EGR device), even if the spark-advance angle is controlled by taking the engine rotational speed and the engine load into consideration, a sufficient optimum spark-advance angle cannot be obtained at all.
In order to eliminate this defect occurring in the conventional technique, the applicant previously proposed a method in Japanese patent application No. 51-81,890 for controlling the spark-advance angle in an internal combustion engine provided with an EGR device by using a digital computer. According to this method for controlling the spark-advance angle, the total amount of gas sucked into the engine is obtained by calculation of the measured rotational speed and intake pressure. The ratio of the amount of intake air to the amount of recirculated exhaust gas (hereinafter referred to as EGR gas), namely the EGR ratio, is calculated from the result of such calculation and the amount of actually measured intake air. An optimum spark-advance angle is determined by using the so calculated EGR ratio, and the ignition timing is controlled based on the so determined optimum spark-advance angle.
However, by using the above-mentioned method, the risk of a serious error occurring in the EGR ratio is high especially at low engine rotational speed since discrepancies and errors readily occur between the calculated amount and the actual amount of the total gas. Such error in the EGR ratio can cause a serious error in the spark-advance angle controlled in proportion to the EGR ratio. In such case, if the determined optimum timing angle is too large or too small, serious problems will be caused in the operation of the engine.